Integrated active photonic device and photodetector

ABSTRACT

An active photonic semiconductor device, such as a laser, optical amplifier or LED, is monolithically integrated with a photodetector. The device includes an optically active region formed on a substrate including a first electrical contact for initiating emission of photons within the optically active region; an optical confinement structure generally defining a principal optical path through the device and through said optically active region; and a photodetector structure formed on the substrate including a second electrical contact displaced from and substantially electrically insulated from the first contact, overlying a part of the principal optical path, for receiving carriers generated by said emitted photons. The photodetector is preferably positioned to cover an intermixed/non-intermixed region close to a facet of the device and also close to the active region of the device. The photodetector is weakly coupled to the optical confinement structure such that a very small proportion of the optical radiation can be monitored without deleteriously affecting the performance of the device.

The present invention relates to monolithic integration of photonicdevices, such as semiconductor lasers and optical amplifiers, withphotodetectors.

Photonic devices, such as semiconductor lasers, optical modulators andoptical amplifiers are widely used in modem telecommunication systems.It is desirable to monitor the optical output of such photonic deviceson chip. This is especially desirable when multiple devices areintegrated onto one chip, and more than one optical device has to bemonitored.

However, it is problematic to control or monitor the output of anoptical device output because the gain of a laser or amplifier can beaffected by a number of factors, including:

-   i) environmental effects, such as temperature, humidity, changes in    wavelength and polarisation etc;-   ii) device degradation, due to crystalline defects, deterioration of    contacts, etc; and-   iii) misalignment of optical coupling elements due to shock, strain,    etc.

At present, bulk detectors and couplers are used to monitor and controla semiconductor laser or amplifier, but these prove to be expensive,lossy and impractical for large scale monolithic integration.

For a semiconductor laser, a photodetector can be positioned at the backfacet of the laser. The facets of a semiconductor laser are typicallycoated with a highly reflective (HR) coating, having a reflectioncoefficient, R, of up to ˜95% on the back facet and an anti-reflection(AR) coating with R˜5% on the front facet. The photodetector can measurethe light escaping from the back facet (R˜95%) and hence monitor thedevice.

For a semiconductor optical amplifier, no facet is available formonitoring by a photodetector, since both the front and back facets areemployed for ingress and egress of optical radiation. Therefore, onesolution, as taught in U.S. Pat. No. 5,134,671, is to employ anintegrated branching waveguide, such as a Y-junction waveguide, to tapoff a fraction of the output power to feed to a photodetector, tomonitor the amplifier.

U.S. Pat. No. 5,134,671 describes a monolithic integrated opticalamplifier and photodetector. The optical amplifier and photodetector areintegrated on the same substrate, the photodetector being opticallycoupled to the optical amplifier via a branching waveguide having lowradiative loss and low back reflectivity. This is achieved with adifficult manufacturing process to form the Y-shaped waveguide with abranch of the waveguide having a gradual decrease in the effectiverefractive index such as to decrease the difference between therefractive indices at the optical interface of the truncated wedge tipto avoid optical coupling of the amplifier.

Due to fabrication/device limitations, practical Y-junction waveguideshave truncated wedge tips. See, for example, Sasaki et. al. ElectronicLetters, Vol. 17, No. 3, pp 136-8 (1989). However, a blunted Y-junctiontip, which inhibits a substantial amount of optical back-reflection tothe optical amplifier, restricts the monolithic integration of a coupledoptical amplifier and monitoring photodetector.

A 1.3 μm laser with an integrated power monitor using a directionalcoupling optical power tap is described in U. Koren et al, IEEEPhotonics Tech. Letters, Vol. 8, No. 3, p 364 (1996). This workdescribes a Y-junction optical tap using a passive dual waveguidedirectional coupler next to the back HR facet of the cavity.

The disadvantage with the process described is that four growth stepsare required to construct the device, including an overgrowth to depositthe passive waveguide region. The different growth steps considerablyincrease the device fabrication difficulty, hence reduce yields andincrease costs.

It is an object of the present invention to provide a photonic devicesuch as a semiconductor laser or amplifier with an integrated photodetection device that is easy to manufacture. It is a further object ofthe present invention to provide such a device in which the interferencewith the optically active lasing or amplifying device by thephotodetection device is reduced over prior art systems.

It is a further object of the present invention to provide aphotodetection device integrated onto the same substrate as a photonicdevice and positioned in relation to a bandgap shifted portion of the deice, that can be used to test the bandgap shift.

According to one aspect, the present invention provides an activephotonic device and photodetector integrated on a single substrate, thephotodetector adapted for monitoring an output of the active device,comprising:

-   -   a semiconductor substrate;    -   an optically active region formed on the substrate including a        first electrical contact thereon for initiating emission of        photons and/or modulation of photons within the optically active        region;    -   an optical confinement structure generally defining a principal        optical path through the device and through said optically        active region;    -   a photodetector structure formed on the substrate including a        second electrical contact displaced from and substantially        electrically insulated from the first contact, overlying a part        of the principal optical path, for receiving carriers generated        by said emitted photons.

According to another aspect, the present invention provides an activephotonic device and photodetector integrated on a single substrate, thephotodetector adapted for monitoring an output of the active device,comprising:

-   -   a semiconductor substrate;    -   an optically active region formed on the substrate including a        first electrical contact thereon;    -   a non-branching optical confinement structure generally defining        an optical path through the device and through said optically        active region;    -   a photodetector structure formed on the substrate including a        second electrical contact displaced from and electrically        insulated from the first contact for receiving carriers        generated by photons in the optically active region.

According to a further aspect the present invention provides an activephotonic device and characterisation contact integrated on a singlesubstrate, the characterisation contact for enabling detection of adegree of bandgap shift in the device, comprising:

-   -   a semiconductor substrate;    -   an optically active region formed on the substrate and        comprising a semiconductor medium having a first bandgap, and        including a first electrical contact thereon for initiating        emission of photons and/or modulation of photons within the        optically active region;    -   a bandgap shifted region formed on the substrate and comprising        a semiconductor medium having a second bandgap shifted from said        first bandgap;    -   a characterisation contact formed on the substrate, displaced        from and substantially electrically insulated from the first        electrical contact, at least a part of the characterisation        contact overlaying the bandgap shifted region.

According to another aspect, the present invention provides a method ofdetermining a degree of bandgap shift introduced between a first regionof semiconductor medium and a second region of semiconductor medium,comprising the steps of:

-   -   forming a photonic device on a substrate, including a first        region in which the semiconductor medium has a first bandgap,        and a second region in which the semiconductor medium has a        second bandgap shifted from said first bandgap;    -   depositing a first contact in said first region for operating        said photonic device;    -   depositing a second contact at least partially overlying said        second region; and    -   electrically biassing said second contact to generate an        electroluminescence signal in the semiconductor medium        indicative of the magnitude of at least said second bandgap.

According to another aspect, the present invention provides a method ofdetermining a degree of bandgap shift introduced between a first regionof semiconductor medium and a second region of semiconductor medium,comprising the steps of:

-   -   forming a photonic device on a substrate, including a first        region in which the semiconductor medium has a first bandgap,        and a second region in which the semiconductor medium has a        second bandgap shifted from said first bandgap;    -   depositing a first contact in said first region for operating        said photonic device;    -   depositing a second contact at least partially overlying said        second region; and    -   optically stimulating said second region to generate        electroluminescence in the semiconductor medium;    -   electrically biassing said second contact so as to draw a        photodetection current indicative of the magnitude of at least        said second bandgap.

Throughout the present specification, the expression “active photonicdevice” is intended to encompass all optically active semiconductordevices deploying electrical charge injection techniques to generatephotons or to modulate photons in an optically active region of thesemiconductor. The invention is particularly suited for monolithicintegration of multiple optical devices on a single chip fortelecommunication applications. However, the invention can be applied tothe monitoring of any active photonic device as defined above, includinglasers, amplifiers and light emitting diodes.

The devices may be formed in any suitable semiconducting medium,particularly III-V and II-VI material systems.

Embodiments of the present invention will now be described by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 shows a schematic transverse cross section of a laser devicehaving a photodetector contact positioned laterally adjacent to anoptical confinement structure of the laser device;

FIG. 2 a shows a schematic top view of the laser device of FIG. 1, inwhich the photodetector contact partially overlies a bandgap shiftedregion;

FIG. 2 b shows a schematic top view of a laser device in which thephotodetector contact is positioned within the optical confinementstructure and partially overlying the bandgap shifted region;

FIG. 2 c shows a schematic top view of the laser device similar to FIG.2 b, but with the photodetector contact positioned at the highlyreflective coating end of the device;

FIG. 2 d shows a schematic top view of the laser device similar to FIG.2 a, but with the photodetector contact positioned at the highlyreflective coating end of the device;

FIG. 2 e shows a schematic top view of the laser device similar to FIG.2 b, but with the photodetector contacts positioned entirely within thebandgap shifted regions;

FIG. 2 f shows a schematic top view of the laser device similar to FIG.2 e, but with the photodetector contacts positioned entirely outside thebandgap shifted regions;

FIG. 3 a shows a schematic diagram of the band gap at the facet end ofthe device of FIG. 1; and

FIG. 3 b shows a schematic diagram of the band gap at the facet end ofthe device of FIG. 1, with the photodetector operating in forward biasmode.

The present invention provides for monolithic integration of an activephotonic device such as a semiconductor laser or optical amplifier and aphotodetector device. The invention describes a simple monolithicsolution to monitor, and hence enable control of, the output power of asemiconductor laser diode. The invention is particularly advantageousfor large scale integration of multiple lasers or optical amplifiers onchip.

With particular reference to FIGS. 1 and 2 a, a semiconductor laser 10comprises an optically active region 11, including a waveguide portion16 extending therethrough. The optically active region 11 provides asemiconductor medium having a suitable band gap, in which carriers maybe injected to create photons or modulate photon behaviour whenoperating in forward bias diode, using techniques well known in the art.

Optically passive regions 12, 15, having an increased band gap, areformed at each end of the waveguide portion 16, preferably using quantumwell intermixing techniques, although any suitable method of locallyincreasing the bandgap is also acceptable.

The intermixed regions 12, 15 (or, more generally, the bandgap shiftedregions) define non-absorbing mirrors (NAMs). On an optical output endof the laser 10, the NAM 12 is provided with an anti-reflective (AR)coating 13, and at the other end of the laser, the NAM 15 is providedwith a high reflectivity (HR) coating 14. The use of intermixed facetends to obtain the NAMs avoids catastrophic optical damage (COD) to thefacets, allowing high power and long lifetime laser diodes to befabricated.

A typical semiconductor laser diode is fabricated by etching thewaveguide portion 16, using conventional processing techniques, as aridge 18. The ridge is typically between 1 and 2 μm in height and widthand of the order of 1000 μm in length. The ridge contains the major partof the optical field distribution 1 and substantially confines theelectrical injection current 2 and 3. However, it will be understoodthat the principles of the invention can be applied in the context ofany suitable optical confinement structure in a semiconductor medium,including buried heterostructures.

A p-type contact 21 is deposited on top of the ridge 18 to facilitatethe current injection into the device 10. An n-type contact 5 isprovided on the bottom of the device on or in the substrate. The body ofthe device is formed in conventional manner with an intrinsic opticallyactive layer 7 generally confined by respective p- and n-type opticallyconducting layers 4 and 6. The p-type optically conducting layer 4 istypically of the order of 200 nm thick. Current is injected across thecontacts 21 and 5; electrons and holes recombine in the optically activelayer 7 to create photons. The ridge 18 constrains the optical mode ofthe device. The geometry of the p-type contact 21 and ridge effectivelydetermine the lateral extent of a principal optical pathway 23 thatpasses through the device 10 between the facets at coatings 13 and 14.

The expression principal optical pathway is used to indicate the pathwaythrough the semiconductor medium in which the substantial part of theoptical field distribution 1 passes, and will be determined by, thoughnot necessarily coextensive with, the optical confinement structure.This is due to the fact that significant leakage of the optical field 1occurs out of the ridge waveguide 18 as shown in FIG. 1.

Preferably, the optical confinement structure, and thus the principaloptical pathway 23, is substantially linear, as shown in the figures.Still more preferably, the optical confinement structure, and thus theprincipal optical pathway, is non-branching. The optical confinementstructure may provide for a single optical mode of operation.

A further p-type contact 22 is deposited laterally separated from theridge contact, to provide a photodetector contact. Preferably, tosimplify the manufacturing process, this further p-type contact 22 isdeposited at the same time as the laser p-type ridge contact 21.

In a preferred embodiment, the photodetector comprises a photodiode, andthis photodiode contact 22 is located sufficiently close to the ridgecontact 21 that there is overlap with the optical field generated by theactive region of the laser. However, the photodiode contact is locatedsufficiently far from the ridge contact to limit current spreading ofthe injection current 2 (see FIG. 1). Thus, the photodetector contact ispositioned such that it at least partially overlies a small part of theprincipal optical pathway through the device, but is laterally separatedfrom the optical confinement structure, eg. ridge 18.

The relative position of the contacts 21 and 22 is such as to ensurethat the optically active device and the photodetector: (a) aresufficiently far apart that there is no serious electrical cross-talkbetween devices; (b) are sufficiently close together that there isenough light to generate a photocurrent and hence signal in thephotodiode; and (c) do not seriously interfere to compromise theperformance of the optically active device, for example by way ofoptical feedback into a laser. In the preferred configuration of FIG. 2a, the lateral separation distance of the contacts 21 and 22 is of theorder of 10 μm.

The photodiode is preferably also positioned at least partly over thepassive (bandgap shifted) region 12 and the active region 11, and closeto the laser output facet 13 as best seen in FIG. 2 a. Although thephotodiode contact 20 is shown in FIG. 2 a at the optical output end ofthe laser (ie. adjacent to the AR coating 13 of the NAM 12), thephotodetector 20 can also be located adjacent to the HR coating 14 ofthe NAM 15, as shown in FIG. 2 d.

With reference to FIG. 2 b, an alternative configuration ofphotodetector 30 is shown. In this embodiment, the contact 31 for thephotodiode 30 is located directly on top of the ridge 18 in longitudinalalignment with, but spaced from, the ridge contact 21. Although thephotodiode contact 31 is shown in FIG. 2 b at the optical output end ofthe laser (ie. adjacent to the AR coating 13 of the NAM 12), thephotodetector contact 35 can also be located adjacent to the HR coating14 of the NAM 15, as shown in FIG. 2 c.

It will be understood that the photodetector can be provided in similarmanner in an optical amplifier in which both ends of the device 10 areprovided with an AR coating.

In the embodiments of FIGS. 2 a and 2 d, the photodetector contact 22,40 is shown laterally offset from the ridge 18. As best viewed in FIG.1, the contact is positioned to overlap the ‘tail’ of the optical fielddistribution, but sufficiently far away from the current injection 2into the active region to avoid significant interference therewith, aspreviously described.

The photo detectors 20, 30, 35, 40 are weakly coupled to the activeregions 16 of the lasers such that a very small proportion of theoptical radiation from the laser active region can be monitored withoutdeleteriously affecting the performance of the laser. In a typicalexample, the ridge contact 21 will supply an injection current in theregion of several hundreds of microamps whereas the photodetectorcontact 22 will only need to draw a detection current in the region ofpicoamaps to nanoamps, ie. a current approximately of the order of10⁴-10⁸ times smaller.

In photodiode mode, the photodiode contact 20 is driven in reverse biasmode such that photons from the ‘tail’ of the optical field 1 cangenerate carriers in the band and hence create a photocurrent which canbe measured. The relative power that is ‘tapped’ out, which determinesthe responsivity of the detector, can easily be controlled by thedistance between the photodiode and ridge.

The advantage of measuring the photocurrent by this method is that thereis effectively no loss to ‘tap-off’ optical power and there is nooptical coupling mechanism between the laser 10 and the detector 20 thatcan create an additional cavity effect that could have a deleteriouseffect on the optical performance of the laser.

Because it is no longer necessary to place a photodetection devicebehind the HR coated facet 14 of the device, the reflectivity of thisfacet may be increased from the conventional figure of R˜95% to amaximum value of R>99.9%. An increase in output power of the device of˜5% is therefore possible.

In the embodiments of FIGS. 2 a and 2 d, the photodiode contact 22, 40has been placed off-set to the side of the ridge, towards the AR coatingof the device, or towards the HR coating 14 of the device. In FIGS. 2 b,2 c, 2 e and 2 f, the photodetector contact 31, 35, 50, 51, 60, 61 isplaced on the optical confinement structure (eg. ridge 18) butlongitudinally separated from the ridge contact to a sufficient distanceto ensure adequate electrical isolation therefrom. The operation of thephotodetector in reverse bias mode is similar to that previouslydescribed in connection with FIGS. 2 a and 2 d, although thephotodetector of course is positioned at or close to the peak of theoptical field distribution 1 in the principal optical path.

In the examples of FIGS. 2 a to 2 d, the photodetector contact ispositioned straddling the bandgap shifted region 12 and the non-shiftedregion 11. This enables the photodetector contact 22, 31, 35 to be usedto inject carriers (using a forward bias mode of operation) into thebandgap shifted/non-shifted regions of the device to monitor theeffectiveness of the intermixing process used to create the bandgapshift.

By driving the photodetector contact in forward bias mode, photons willbe generated at first and second wavelengths corresponding respectivelyto the bandgap shifted and non-shifted regions 12 and 11. If the laserdevice 10 is not operational, it is then possible to use an externalphotodetection device to observe the electro-luminescence (EL) signalgenerated, which can be received via the facet at AR coating 13. Therelative separation of the two wavelengths of the EL signal provides ameasure of the degree of bandgap shifting between the two regions 11 and12. In the case of contact 40 (FIG. 2 d), an EL signal generated may bereceived via the facet at HR coating 14 if this allows sufficientoptical transmission for detection purposes. Of course, in an opticalamplifier, this facet would have an AR coating.

This EL signature can provide an in-situ characterisation technique tomeasure the size of the intermixed regions during the manufacturingprocess.

FIG. 3 a shows a schematic of the band-gap of the facet ends of thedevice of FIGS. 2 a to 2 f. The photodiode contact 22, 31, 36 or 40 islocated overlapping the bandgap shifted (intermixed) region 12 or 15 andthe non-shifted region 11 of the device 10 in a passive section of thedevice spaced apart from the active region contact 21. Photons generatedin the optically active region 11 of the device cause correspondingelectron and hole currents 32, 33 that can be measured by thephotodetector 20, 30.

Alternatively, in FIG. 3 b, the photodiode contact 22, 31 or 36 is showndriven in forward bias mode to create carrier current 37, 38 to generateelectroluminescence in the bandgap shifted/non-shifted regions of thedevice.

In the described forward bias mode of operation of the photodiode, anelectrical current is injected to generate an electroluminescencesignal. In a further mode of operation, an external optical source canbe used to stimulate emission of photons of different wavelengths fromthe bandgap shifted/non-shifted regions. The photodetector contact canthen be operated in reverse bias mode, again to detect photocurrentscorresponding to each of the bandgap shifted and non-shifted regions inorder to determine a degree of quantum well intermixing during thefabrication process.

In will be understood that this optical stimulation and reverse biasphotodetection mode of operation can be effected on an uncleaved waferand therefore provide for characterisation of the QWI manufacturingprocess for each laser device fabricated on the wafer.

With further reference to FIG. 2 f, the photodetector contact 60, 61 hasbeen placed wholly within the optically active region 11, on the ridgeeither at the AR coating end, or the HR coating end, or both. In thisarrangement, for use in the reverse bias photodiode mode, thephotodetector contact must be positioned sufficiently far from the ridgecontact to achieve adequate electrical isolation. Those skilled in thefield will appreciate that electrical isolation may be particularlyeffected by inclusion of an electrical isolation structure in thesemiconductor medium between the two contacts 21 and 60 or 21 and 61. Bycontrast, in the embodiment of FIG. 2 e, adequate electrical isolationis assured by the bandgap shifted region in which the photodetectorcontact 50 or 51 resides.

In all of the described embodiments, the p-type metallisation of theridge contact 18 and adjacent photodetector contact 22, 31, 36, 40, 50,60 etc can be deposited simultaneously to improve the manufacturabilityof the device.

It will be understood that the exact configuration and location of theactive photonic device with respect to the photodiode is dependent onthe particular application. For example, a higher power laser diodewould require a photodiode with the same level of detectivity as for alow power laser and thus can be positioned further away from the laser.The responsivity of the detector can be of the order of 0.1 mA/mW orless. The responsivity of the detector can be changed by varying thedistance from the optical source. If the distance between the activephotonic device and photodetector is short such that electricalcross-talk could occur, then electrical isolation can be obtained byusing conventional isolation techniques, such as a shallow etch and/orion implantation.

Aspects of the invention provide the following advantages over the priorart devices.

1) the diode contact is deposited adjacent to (and preferably at thesame time as) the p-type ridge contact 18. Thus, there need be noadditional processing steps than used in making a laser or amplifier.

2) The photodetector 20 can be fully integrated with multiple laserdevices on the same chip.

3) There is no need to fabricate a complex Y-junction waveguide.

4) Since manufacture of the device is by ‘on-chip’ processing, thereliability of the device will be improved over those that requireattachment of discrete diode components.

5) The packaging process is simplified, thereby reducing manufacturingcost.

6) The device can operate as a photodiode to monitor the optical poweron the back facet and/or the front facet.

7) The device can monitor the effectiveness of the NAM by operating inreverse bias. The EL emission measurement can determine the band-gapshift.

8) The back reflector can have a reflectivity value of up to 99.9%.Therefore the forward output power can be increased by approximately 5%over devices which position a photodetector behind the back facet.

9) The photodetector does not significantly influence the performance ofthe active photonic device.

Although the preferred implementation of the photodetector describedabove is in conjunction with an active device having an opticalconfinement structure for operating in a single optical mode ofoperation, the principles can also be applied to multimode devices,optical amplifiers and light emitting diodes.

Other embodiments are intentionally within the scope of the accompanyingclaims.

1. An active photonic device, comprising: a semiconductor substrate; anoptically active region formed on the semiconductor substrate, theoptically active region comprising a first electrical contact forinitiating emission of photons and/or modulation of photons within theoptically active region; an optical confinement structure defining aprincipal optical path through the active photonic device and throughthe optically active region; and a photodetector structure formed on thesemiconductor substrate, the photodetector comprising a secondelectrical contact displaced from, and substantially electricallyinsulated from, the first electrical contact and overlying at least partof the principal optical path, the photodetector for receiving carriersgenerated by emitted photons.
 2. The active photonic device of claim 1,wherein the optical confinement structure is substantially linear. 3.The active photonic device of claim 1, wherein the optical confinementstructure is non-branching.
 4. The active photonic device of claim 1,wherein the optical confinement structure has a single optical mode. 5.The active photonic device of claim 1, wherein the second electricalcontact is positioned on the semiconductor substrate laterally offsetfrom an axis of the principal optical path.
 6. The active photonicdevice of claim 1, wherein the second electrical contact is positionedon the semiconductor substrate directly over an axis of the principaloptical path and longitudinally separated from the first electricalcontact.
 7. The active photonic device of claim 1, further comprising atleast one bandgap shifted region in the optical path, the at least onebandgap shifted region having a larger band gap than the opticallyactive region, wherein the second contact is positioned at least partlyover the bandgap shifted region.
 8. The active photonic device of claim7, comprising a bandgap shifted region at each end of the optical path.9. The active photonic device of claim 1, wherein the opticalconfinement structure comprises a ridge waveguide.
 10. The activephotonic device of claim 7, wherein bandgap shifted region is formedusing at least one intermixing technique.
 11. The active photonic deviceof claim 1, wherein the active photonic device comprises a laser, andthe optical confinement structure comprises a mirror at one end thereofhaving a reflection coefficient of substantially higher than 95%. 12.The active photonic device of claim 11, wherein the mirror has areflection coefficient of greater than or equal to 99.9%.
 13. The activephotonic device of claim 1, further comprising an electrical isolationstructure positioned between the first and second electrical contacts.14. An active photonic device, comprising: a semiconductor substrate; anoptically active region formed on the semiconductor substrate, theoptically active region comprising a first electrical contact; anon-branching optical confinement structure defining an optical paththrough the active photonic device and through the optically activeregion; and a photodetector structure formed on the semiconductorsubstrate, the photodetector structure comprising a second electricalcontact displaced from, and electrically insulated from, the firstelectrical contact, the photodetector structure for receiving carriersgenerated by photons in the optically active region.
 15. An activephotonic device, comprising: a semiconductor substrate; an opticallyactive region formed on the semiconductor substrate and comprising asemiconductor medium having a first bandgap, the optically active regioncomprising a first electrical contact for initiating emission of photonsand/or modulation of photons within the optically active region; abandgap shifted region formed on the semiconductor substrate, bandgap,shifted region comprising a semiconductor medium having a second bandgapshifted from the first bandgap; and a characterization contact formed onthe semiconductor substrate and displaced from, and substantiallyelectrically insulated from, the first electrical contact, at least apart of the characterization contact overlying the bandgap shiftedregion, the characterisation contact for enabling detection of a bandgapshift in the active photonic device.
 16. The active photonic device ofclaim 15, wherein the characterization contact entirely overlies thebandgap shifted region.
 17. The active photonic device of claim 15,wherein the characterization contact is laterally offset from the firstelectrical contact.
 18. The active photonic device of claim 15, whereinthe characterization contact is adjacent to an output facet of theactive photonic device.
 19. A method of determining a degree of bandgapshift between a first region of semiconductor medium and a second regionof semiconductor medium, the method comprising: forming a photonicdevice on a substrate, the photonic device comprising a first region inwhich the semiconductor medium has a first bandgap and a second regionin which the semiconductor medium has a second bandgap shifted from saidfirst bandgap; depositing a first contact in the first region foroperating the photonic device; depositing a second contact at leastpartially overlying the second region; and electrically biasing thesecond contact to generate an electroluminescence signal in thesemiconductor medium, the electroluminescence signal being indicative ofa magnitude of at least the second bandgap.
 20. The method of claim 19,wherein the second contact is deposited overlying portions of both thefirst region and the second region, and wherein the electroluminescencesignal is indicative of a difference in magnitude between the first andsecond bandgaps.
 21. A method of determining a degree of bandgap shiftbetween a first region of semiconductor medium and a second region ofsemiconductor medium, the method comprising: forming a photonic deviceon a substrate, the photonic device comprising a first region in whichthe semiconductor medium has a first bandgap and a second region inwhich the semiconductor medium has a second bandgap shifted from saidfirst bandgap; depositing a first contact in the first region foroperating the photonic device; depositing a second contact at leastpartially overlying the second region; optically stimulating the secondregion to generate electroluminescence in the semiconductor medium; andelectrically biasing the second contact so as to draw a photodetectioncurrent indicative of a magnitude of at least the second bandgap. 22.The method of claim 21, wherein the second contact is depositedoverlying portions of both the first region and the second region, andwherein the photodetection current generated is indicative of adifference in magnitude between the first and second bandgaps.